How Many Water Molecules Are in Your Body?
The human body is about 60% water by weight. If you weigh around 185 pounds (~84 kg), that means:
- ~50 liters of water, or
- ~50,000 grams of water
How Many Molecules Is That?
One mole of water (18 grams) contains ~6.022 × 10²³ molecules (Avogadro’s number). So:
- 50,000 grams / 18 grams per mole = ~2777.8 moles
- 2777.8 moles × 6.022 × 10²³ = ~1.67 × 10²⁷ molecules
That’s roughly 1.67 octillion water molecules — a 1 followed by 27 zeros.
Has That Water Been in Other Living Beings?
Yes — almost certainly. Water on Earth is continuously cycled through the hydrologic cycle: evaporation, condensation, precipitation, ingestion, and excretion.
Since water molecules aren’t typically destroyed, over time they’ve moved through countless organisms. It’s almost certain that millions or billions of the molecules in your body once passed through:
- Other people
- Animals
- Plants
- Even dinosaurs
There’s a fun calculation that suggests: Every glass of water you drink likely contains at least one molecule that passed through Cleopatra or Einstein.
Can Water Molecules Be Broken Apart?
Yes — but it’s rare. Some examples include:
- Photosynthesis in plants splits H₂O into hydrogen and oxygen.
- Cellular respiration can also alter water molecules.
Still, most water molecules stay intact as water for long periods of time.
The Takeaway
- Your body contains over 10²⁷ water molecules.
- Trillions of them were almost certainly in other life forms — people, animals, plants, even dinosaurs.
- Water is part of a shared, ancient planetary cycle that connects us all.
Every sip of water is a sip of history.